Thermoplastic materials are those that soften and flow upon application of pressure and heat. Thermoplastic foams are defined, generally, as foams made from thermoplastic resins. Because thermoplastic materials regain their original properties upon cooling, most thermoplastic materials can be remolded many times. Examples of thermoplastic resins include poly(vinyl chloride), polyethylene polystyrene, acrylate resins, and poly(ethylene terephthalate).
It is known to produce thermoplastic resin foam materials having substantially closed-cell structures by intimately incorporating within the resin material a volatile organic liquid which vaporizes upon heating to form a gas (the liquid is known as the blowing agent, its resultant vapor the blowing gas). It is also known to use a solid substance as the blowing agent where the solid substance decomposes to form the blowing gas. The vapor created from the blowing agent is the blowing gas (often also referred to as simply the blowing agent) and causes the thermoplastic to expand and form a cellular mass.
Thermoplastic resin materials that have been foamed by the action of a volatile organic blowing agent producing a primary foaming gas may thereafter be induced to further expand. This secondary expansion is achieved by exposing the foamed material to another gas (a secondary gas), such as steam or air, which has a permeability rate greater than the permeability rate of the primary foaming gas through the cell walls of the foamed mass. During the exposure to this secondary gas, the material is reheated to a heat softening temperature. The secondary gas, which has a permeability rate through the cell wall greater than that of the primary gas already in the cell, permeates the cell wall and joins the primary gas inside the cell. At the heat softening temperature, the combined effect of the primary gas and the secondary gas causes further expansion of the initially foamed material. The result is a lower density foam product.
It is further known that thermoplastic resin materials that have been foamed by the gas emitted upon decomposition of a solid substance may thereafter be induced to further expand. This further expansion is achieved by heating the foamed material to a temperature near the melting point of the resin while subjecting it to a secondary gas at superatmospheric pressure. After this step is performed, the foamed material is reheated to a heat softening temperature at a lower pressure (i.e. atmospheric pressure). This causes the gas to expand inside the cells. The combined expansion of the primary gas and the secondary gas (which has entered the cells of the foamed material primarily because of the internal/external pressure differential during the application of the superatmospheric pressure) produces a lower density foam product.